In U.S. Pat. No. 3,895,054 there is disclosed a process for the manufacture of urethanes (carbamic acid esters) by reacting an organic compound containing at least one hydroxyl group with carbon monoxide and a nitrogenous organic compound at elevated temperature and pressure in the presence of a selenium catalyst and a base and/or water.
The present invention is directed to an effective method of recovering selenium from urethane solutions produced, for example, by the above described process and containing selenium which may be in the form of metallic selenium, inorganic selenium compounds or organoselenium compounds. Because of the high reactivity of selenium, when used as a catalyst as in the above reaction its combination with organic compounds very frequently results in the formation of organoselenium compounds which remain in solution with the urethane product. In such reaction, a portion of the selenium catalyst such as metallic selenium per se or selenium dioxide, selenium disulfide, selenium oxychloride, etc., is converted to one or more organoselenium compounds which may be classified generally as selenols, selenides, benzoselenazoles, esters of selenocarbonic acid, selenic acid and selenious acid, selenones and the like. The type and number of organoselenium compounds which may be formed is a function of the reaction conditions used to produce the urethanes such as time, temperature, pressure and solvent.
Because of the cost and toxicity of selenium, it is essential that as much of the selenium be recovered from the urethane reaction product as is possible and from the organoselenium compounds in a form suitable for reuse as a catalyst.
A number of prior art processes have been proposed for the recovery of selenium from various organic reaction products or for the deselenation of organoselenium compounds. However, such prior art processes are generally narrow in scope and application and have proven to be of little or no value to the recovery of selenium from urethane solutions containing same. In addition, such processes do not provide for recovery of the metallic selenium for reuse.
U.S. Pat. No. 3,048,604 discloses a deselenation of dehydrosteroids containing selenium by reaction of the contained selenium compound with copper powder at temperatures of from 100.degree. to 300.degree. C.
In an article by H. Hauptmann and W. F. Walter, Journal of the American Chemical Society, Vol. 77, pp. 4929-4930, Sept. 20, 1955, the action of Raney Nickel on organoselenium compounds to effect deselenization is described. A large excess of the Raney Nickel is necessary accompanied by long reaction times resulting in partial deselenization.
U.S. Pat. No. 3,084,994 discloses a method for the recovery of selenium from a gas containing selenium together with aldehydes and nitriles using water sprays to collect the selenium and atomizing the selenium-containing water in molecular oxygen-containing gas in contact with an oxide of copper, iron or nickel at 500.degree. C. to convert the selenium in the water solution to selenium dioxide.
U.S. Pat. No. 3,577,216 discloses a process for the recovery of selenium IV used as a catalyst in the oxidative production of carboxylic acids by adding to the reaction products magnesium, zinc, aluminum, alkali and alkaline earth metal hydroxides, oxides, carbonates, bicarbonates and salts of the metals to precipitate metal selenite and reacting the metal selenite with a strong inorganic acid to obtain one of selenious acid and anhydride.